Ship stabiliser automatic gain control system

ABSTRACT

An improved fin stabilizer control system which has an automatic gain control (AGC) system which uses the difference between the control laws demand signals and the actual fin angles to calculate a gain factor to reduce the control laws demand until the non-linearities almost disappear. If more than one control law is used to generate a demand, the control laws can be prioritized and the available actuator power thus rationed. In particular, the automatic gain control (AGC) sub-system of the control law monitors the motions of the fins and detects when each fin is not following the control law demands. It adjusts the gain of the control law so as to reduce the slew rate limit and the fin angle limit effects. Although the automatic gain control (AGC) sub-system has the effect of reducing the control law loop gain, it reduces the ship motions below those achieved with a conventional control law because the phase lag between the demanded and actual fin angles is reduced and because generation of harmonics of the demanded fin angle is much reduced.

The present invention relates to a system for stabilising the movementof ships or floating vessels. In particular the invention relates to afin stabiliser control system for use with ships.

A fin stabiliser control system is used to control stabilisers of a shipin order to reduce ship motions. For mono-hulls, because of the forcesand moments required, roll stabilisation only is normally used. Forsemi-submersibles, catamarans, or SWATH (Small Water-Plane Area TwinHull) vessels control of other motions such as pitch or heave may alsobe practical.

Usually fins and their associated actuation systems exhibit two majornon-linearities; slew rate limiting and fin-angle limiting. The firstnon-linearity; slew rate limiting, occurs because the actuation systemsare only capable of drive the fins up to some finite rate. This rate mayvary with the fin angle. The second occurs if the fin is operated abovea certain angle, when "stalling" of the fin occurs.

This behaviour adversely affects control law performance and this can beexacerbated if the non-linearities generate intermodulation (coupling)between multiple control laws.

Existing stabiliser controllers employ fixed gain. This means as seadisturbances increase, the fin demand increases until it is limited.Under severe overload the fin spends its time either at the nose-up ornose-down limit position or slewing at its maximum slew rate between thenose-up and the nose-down limit positions. Because the maximum fin slewrate is finite, the effect when slew rate limiting occurs is that thefin position is delayed with respect to the fin demand. The slower theslew rate, the more pronounced this effect. The delay has the effect ofreducing the effectiveness of the roll stabiliser control loop byintroducing a phase delay, and the effectiveness of control law reducesrapidly with increasing phase delay.

There are two possible ways of dealing with this problem. The first wayis to ensure that the maximum slew rate of the fin is such that thephase delays introduced for all reasonable disturbances are low enoughnot to matter. This has the advantage of simplicity, but thedisadvantage what the drive system must be powerful enough, and providesufficient torque, to provide the requisite slew rates.

The second way is to reduce the controller loop gain so that the finslew rate is never exceeded and the nose-up and nose-down limits arenever reached. However, as the control gain is reduced, thenon-saturated roll reduction reduces as well. In this situation theprincipal advantage is that the drive system, power and torquecapability may be reduced considerably when compared with the firstpotential solution.

In designing a suitable fin stabiliser controller, the main points toconsider are the relative fall off in performance in the two types ofpotential solution with increase in disturbances for a given drivesystem power and torque capabilities, and also the trade off betweenperformance and cost. In this regard, it is desirable to reduce the costof the drive system whilst not comprising the roll reduction achieved.

An object of the present invention is to provide an improved finstabiliser control system which obviates or mitigates at least one ofthe aforementioned disadvantages.

This is achieved by providing an automatic gain control system whichuses the difference between the control laws demand signals and theactual fin angles to calculate a gain factor to reduce the control lawsdemand until the non-linearities almost disappear.

If more than one control law is used to generate a demand, the controllaws can be prioritised and the available actuator power thus rationed.In particular, the automatic gain control (AGC) sub-system of thecontrol law monitors the motions of the fins and detects when each finis not following the control law demands it adjusts the gain of thecontrol law so as to reduce the slew rate limit and the fin angle limiteffects. A control law is a physical realisation of a mathematicalequation which continuously generates actuater demands in response toerror inputs in order to achieve the desired motion or positioncharacteristics of an object, in this case the fin or fins. The controllaw may be identical in structure to current control laws or of adifferent structure provided that it can accommodate changes to the loopgain.

Although the automatic gain control (AGC) sub-system has the effect ofreducing the control law loop gain, it reduces the ship motions belowthose achieved with a conventional control law because the phase lagbetween the demanded and actual fin angles is reduced and becausegeneration of harmonics of the demanded fin angle is much reduced.

The AGC sub-system solves the two basic problems. It firstly measuresthe level of slew rate limiting and/or fin angle limiting by subtractingthe demanded fin angle from the actual fin angle and the absolute valueof the result is compared with the corresponding absolute valuescalculated from all other fins the stabilisation system, and the largestof these values is selected and filtered to produce an overloadmeasurement. The filter used may have different "attack" and "decay"times. Therefore, if an overload occurs it may cause a rapid increase inthe overload measurement, while if such an overload is removed thereturn to lower overload measurements is much slower. This preventsoverload from being present for long periods while preventing rapidmodulations of control law gain.

The overload measurement is then converted to a gain which is variedfrom some maximum (for no overload), to a selectible minimum (for largeoverloads). In practice, the minimum is set to a suitable value (forexample 30% of the maximum) so that the control law gain is notexcessively reduced and the slope of the gain against overload is madevariable to allow the AGC sub-system to be tuned for a particularconfiguration. The control law fin angle demand is then multiplied bythe gain value calculated above to derive the actual fin angle demand.

For fin stabilisers, the instantaneous power consumption is proportionalat high slew rates to the square of the slew rate. Thus the AGCcontroller can reduce the average power consumption by reducing theamount of time the fin operates at high slew rates. Alternatively, itwill be understood that the AGC controller can be used with lower slewrate fins. This slew rate can be selected to provide the sameperformance levels as current fins with conventional controllers, butwith much lower average and peak Dower consumption.

According to one aspect of the present invention there is provided avessel motion stabilisation system for stabilising at least one motionof a vessel having a plurality of actuatable vessel stabilisingelements, said stabilising system comprising:

motion sensing means for sensing a parameter representative of thevessel motion to be stabilised,

control law means for receiving said motion signal and for processingsaid motion signal to provide an unlimited stabilising element demandsignal,

automatic gain control means for receiving a signal representative ofsaid unlimited control law demand signal and for receiving a signalrepresentative of said actual stabilising element condition,

said automatic gain control element including means for comparing theunlimited demand signal with the actual demand signal and providing anerror value, and signal processing means for processing said error valueto provide automatic gain control value,

means for combining the gain control value with the unlimited demandsignal to provide a limited or actual demand signal for controlling themotion of said motion stabilising element.

Preferably, said automatic gain control means includes absolute errordetecting means for determining the absolute error between thestabiliser demand signal and the actual stabiliser signal, and filtermeans coupled to the output of the absolute error detecting means toprovide a filtered response loop gain control means for receiving theoutput of the filter means for limiting the loop gain of the controlunit to a value between a maximum and a minimum.

Conventionally the vessel is a ship and the vessel stabilising elementsare fins which are hydraulically or electrically actuatable. The finmotion detecting elements are preferably accelerometers, rate sensors orangle sensors, and the motion controlled is the roll motion of the ship.

Preferably, said loop gain control means limits the maximum demand angleto a pro-determined value, for example, plus or minus 25°.

According to another aspect of the present invention there is provided avessel stabilising system for stabilising the motion of a vessel, thevessel having a plurality of vessel motions to be controlledsimultaneously, and the vessel having a plurality of actuatable vesselstabilising elements, each vessel stabilising element having position ormotion sensing means for providing an output signal representative ofthe position or motion of the stabilising element, said vesselstabilising system comprising:

signal weighting means coupled to said motion sensing means forreceiving said output signals representative of the position or motionof said stabilising elements, said weighting means including means forweighting the proportion of available power to be set for a particularvessel motion control law according to pre-determined criteria,

error detecting means coupled to each stabilising element for receivingand coupling a demand signal and an actual signal from each vesselstabilising element and for providing an error signal,

error signal comparison means coupled to each of said error detectingmeans for receiving said error signals and for coupling said errorsignals and selecting the largest error signal as an output signal,

output signals processing means for filtering and amplifying said outputsignal to provide a gain value between a maximum and a minimum,

power allocation means coupled to the output of said weighting means andto said signal processing means to receive said weighted vessel motioncomponents and said gain value, said power allocation means-includingsumming means for summing said weighted vessel motion components toprovide a total value, and a plurality of vessel motion gain calculatingmeans for determining respectively the gain of one of said respectivevessel motions, said gain calculating means receiving said total valueof the weighted components, the weighted component for a particularmotion and the gain value between a maximum and a minimum, each gaincalculating means providing a motion gain value which is fed to means todrive said motion stabilising element.

Preferably the vessel is a ship and the vessel stabilising elements arefins. Conveniently, there are four fins which are used by three controllaws.

An accelerometer or other motion sensor provides the motion signal fromeach controlled motion. The motion signals are weighted in accordancewith control laws for each motion which depend on the vessel andinstallation,specific criteria. Weighting is achieved by constants whichare set at the design stage. The constants can be changed in-situ tomeet changing performance requirements.

Conveniently, the maximum is 1.0 and the minimum is zero.

According to another aspect of the invention there is provided a methodof controlling at least one motion of a vessel using a vesselstabilising system, the vessel having a plurality of vessel motions andvessel stabilising elements, said method comprising the steps of,

providing an automatic gain control unit to monitor a signalrepresentative of the motion of each stabilising element to becontrolled and coupling the signal with a desired motion, and

adjusting the gain for each motion signal by reducing the gain to alevel which the stabilising element can achieve.

Preferably the method includes controlling a plurality of vessel motionsby monitoring signals representative of the motions of each stabilisingelement, and processing signals representative of each motion togenerate weighted gain values and drive said stabilising elements tominimise said vessel motions.

These and other aspects of the invention will become apparent from thefollowing description when taken in combination with the accompanyingdrawings in which:

FIG. 1 is a plan view of SWATH vessel with four stabilising fins whichrequire to be controlled by the automatic gain fin stabiliser controllerin accordance with an embodiment of the present invention;

FIG. 2 is an enlarged end view of the Swath shown in FIG. 1 taken in thedirection A.

FIG. 3 is a schematic block diagram of the fin stabiliser control systemused to produce a fin angle demand for roll control law;

FIG. 4 is a schematic block diagram of the automatic gain controllerused with the fin stabiliser control system shown in FIG. 3 for the rollcontrol motion;

FIG. 5 depicts a block diagram of an automatic gain control system forthe SWATH vessel in which 4 actuators are analysed to derive the gainsfor 3 control laws;

FIG. 6 is a schematic block diagram of an overload block shown in FIG.5;

FIG. 7 is a more detailed diagram of the MAX-block shown in FIG. 5 whichcompares all of the error outputs of the overload blocks and detects thelargest so that the AGC is always reacting to the worst case:

FIG. 8 is more detailed schematic block diagram of the filter shown inFIG. 5;

FIG. 9 is a schematic block diagram of the weighting system for allowingthe proportion of available actuator power to be set for each controllaw;

FIG. 10 depicts a schematic block diagram of the power allocator shownin FIG. 5 which sums all of the weighted motion components and passesthe total to the gain calculator; and

FIG. 11 is a schematic block diagram of the cain calculator shown inFIG. 10.

Reference is first made to FIGS. 1 and 2 of the drawings which depicts aplan view of a SWATH, generally indicated by reference numeral 10, whichhas four identical moveable fins 12, 14, 16 and 18 located on the hullas shown. In the following description reference will be made to thecontrol of one of these fins, although it will be appreciated that thecontrol is applicable to all fins in an identical manner using theautomatic gain control system of the present invention. Roll control isdiscussed initially to simplify understanding and the heave and pitchcontrol is discussed in the fin stabilisation system.

Reference is now made to FIG. 3 of the drawings which is a block diagramof the roll control law for processing a vector of ship roll angle, rollvelocity and roll acceleration to produce a fin angle demand. The rollcontrol law will vary from vessel to vessel, but in its simplest formmay be represented by velocity multiplied by a constant and implementedin hardware or software. It will be understood that in practice one ormore of these components may be absent.

The vector, O_(V), of ship roll acceleration velocity and angle is fedto the roll control unit 20 which is implemented in software. Theoutput, O_(D), of the unit 20 is fed to a multiplier 22 which is alsoimplemented in software which multiplies O_(D) by the gain value O_(G),as will be described. The output of multiplier 22, the roll control lawfin angle demand F_(D), is fed to the automatic gain control (AGO) 24and to a limit block 26, also implemented in software, which limits thefin angle demand F_(D) to maximum working angle of the fin (e.g. 25°),designated F_(DL), before being fed to the fin service system (notshown) which continuously attempts to place the fin at the demanded finangle. Thus, it will be appreciated that the actual fin angle demandF_(DL) is a scaled version of the roll control law angle demand F_(D)with the scaling factor being determined by the AGC unit 24.

Reference is now made to FIG. 4 of the drawings which depicts the AGCunit 24 which monitors the difference between the unlimited fin angleF_(D) demand and the actual fin angle F_(A). The actual value F_(A) issubtracted from the fin angle demand F_(D) to provide an error value. Ifthe actual fin angle F_(A) limits or if the slew rate limiting occurs,then the error will increase. The absolute value of this value is takenby block 28, which is implemented in software, so that the sense of theerror does not matter. The resulting absolute value is filtered infilter 30 so that the bandwidth of the control law gain adjustment canbe tailored. Filter 30 consists of a filter loop having an AGC filtertime constant unit 32 over an integrator 34. The time constant unit 32has two time constants to provide a time-averaging asymmetric responseto ensure that AGC operation is smoothed. Normally this bandwidth isconsiderably smaller than the control law bandwidth. The filtered valueis a measure of the overload experienced by the fin and its actuationsystem over the last few cycles of operation. The filter 30 isimplemented in software.

Once the overload measurement has been taken it is then multiplied by again K_(agc) in software-implemented gain unit 36 which is used totailor the loop gain of the AGC sub-system. The loop gain is thenlimited in a software-implemented limit unit 28 to between a minimum(0.0) and a maximum (1.0) and subtracted from 1.0 by the summationsystem indicated Thus, it will be appreciated that as the overloadmeasurement gradually increases, the AGC gain value gradually decreasesfrom 1.0 reaching 0.0 under extreme overload conditions.

It will be appreciated that the roll control law gain is variedaccording to the amount of difficulty of the fin and its actuatingsystem have in following the roll control law demands.

Reference is now made to FIG. 5 of the drawings which depicts a blockdiagram of an automatic gain controller 39 for use in a system where the4 fins (actuators) of the Swath are analysed to derive the gains for 3control laws to control roll, pitch and heave motions. The fins operateas opposed pairs to reduce the roll, pitch and heave. The AGC controllershown in FIG. 5 includes enhancements over those shown in FIGS. 3 and 4to allow for individual selection of control laws and to cater forparticular fins being unavailable.

The AGC system shown in FIG. 5 uses the difference between the controllaws demand and the actual fin angles as described with reference toFIGS. 3 and 4 to calculate multiple gain factors to reduce the totalcontrol laws demand until the non-linearities almost disappear as willbe described. If more than one control law is generating the demand,then the demands can be prioritised and the available actuator power canbe rationed accordingly.

Still referring to FIG. 5, it will be understood that this Figure showstwo logical halves: the lower half generates a single gain reductionfactor K_(agc) based on the fin position errors, and the upper halfallocates fin power between the 3 control laws according to the weightedSwath vessel motions, as will be later described in detail.

Each fin 10, 12, 14 and 16 produces a demand and an angle which is fedto a respective overload unit 40, 42, 44, 46 one of which is seen inFIG. 6. Each overload unit is implemented in software and subtracts theactual fin position from the desired position and passes the differencethrough a deadband unit 47 which sets the difference to zero if it isless than a predetermined deadband value (which is set depending on theapplication). This has the advantage in that it allows for transducercalibration errors. Once this has been achieved the absolute value ofthe difference is taken by block 49, i.e. the sign is discarded. Theoutputs 40a, 42a, 44a and 46a of each of the overload units is fed to asoftware-implemented block 48 designated MAX which compares all of theerror outputs of the overload blocks and then selects the largestabsolute error. This is so that the AGC 39 always reacts to the worstcase. The largest error selected is fed to a filter 50 has a timeconstant unit 51 with two time constants similar to unit 32 and anintegrator 53 which provides a time-averaging asymmetric (that is, afast attack and slow decay) response to ensure that the AGC operation issmoothed. The output of the filter 50 is fed to the gain unit 52 K_(agc)of the AGC gain controller. The value of K is chosen so that thegreatest possible actuator error is converted to unity. The output ofgain unit 52 varies in time from zero, when all of the fins arecorrectly responding to the control law demands, to 1.0 which occurswhen one or more of the fins is at one end of its travel and the controllaw expects it to be at the other end. The output 54 from the gain unit52 is fed to the power allocation block 56 which is best seen in FIG.10.

The power allocation block 56 also receives another input 58, the threemotion components, which originates from the motion sensors 60 which areaccelerometers. The accelerometers provide signals which are integratedto give a time-averaged value (RMS) of velocity or other estimate of therelevant motion and the averaged values are fed to the weighting unit,generally indicated by reference numeral 62.

The weighting unit, as shown in FIG. 9, allows a proportion of availableactuator power to be set for each control law according toinstallation-specific criteria. For example, to reduce the probabilityof sea-sickness the vertical acceleration of passenger areas should bereduced. It will be understood that the worst case is any corner of thevessel where the lever arms due to roll and pitch were greatest. Thusthe velocity value from the roll sensor is multiplied by half the vesselbeam (K_(roll)) and the pitch sensor outputs is multiplied by half thevessel length (K_(pitch)). Because lower vessel motion periods meanhigher accelerations, velocity values are multiplied by the reciprocalsof the periods. Therefore, if the vessel pitch is 10 seconds and theroll period is 20 seconds, then the multiplication values are T_(roll)=0.05 and T_(pitch) =0.1. It will be appreciated that additionalconcepts can be included to further fine tune the AGC performance. Asseen in FIG. 9 the output of the weighting block generates threeweighted motion components corresponding to roll, pitch and heave. Theseweighted components are fed to the power allocator shown in FIG. 10. Theconstants K,T are software-implemented and are set at the design stage,but can be varied on-site to suit particular sea or vessel requirements.

Reference is now made to FIG. 10 and it will be appreciated that thepower allocator 56 receives all three of the weighted motion componentswhich are summed in unit 64 to generate a total value which passed toeach of three motion gain calculators 66a, b and c. The motion gaincalculators 66a, b and c also receive the same overload signal, and eachreceives its respective motion component. This means that, for example,gain calculator 66a receives motion component 1, the total of theweighted motion components and the overload signal.

Each motion gain calculator 66a, b and c is identical and one 66a willbe now be described with reference to FIG. 11. It issoftware-implemented and processes the three input signals by dividingthe sum total by the motion component and then multiplying the outputresult 68 by the overload signal. The result of this multiplication islimited to 1 in a limit unit 70 and, as shown in FIG. 4, and the resultis then subtracted from 1.0 to produce a particular AGC motion gainvalue 72. This is repeated for each motion gain calculator.

Thus, in summary it will be appreciated that the AGC sub-system of thecontrol law monitors the motions of the fin and detects when the fin isnot following the control law demands. It then adjusts the gain, byreducing it to a level that the fin can achieve, of the control law soas to reduce the slew rate limit and fin angle limit effects. Thecontrol law may be identical in structure to current control laws or ofa different structure provided it can cope with having its loop gainchanged from time to time.

It will be understood that in the case of the SWATH ship, more than oneship motion is controlled simultaneously. For example, pitch, heave androll are all controlled simultaneously. This is achieved by firstlyselecting the stabilisation criterion mixed with the contribution of aparticular motion of the ship as measured with respect to thestabilisation criterion. For example, the vertical velocity due to rollat any point of the ship is the roll velocity times the lever arm fromthe centre of roll at the point at which the total vertical velocity isbeing assessed. Finally, the contributions are added together to providea measure of total motion with respect to the stabilisation criterion.The relative gains of the control laws are then calculated as a ratio ofthe contribution from corresponding motion over the total motion. Whenthis is achieved it is possible to weight the individual contributionsbefore adding them together. This is used for instance, to compensatefor coupling between different ship motions.

It will be appreciated that various modifications may be made to theembodiments hereinbefore described without departing from the scope ofthe invention. For instance, in a normal fin service system there arealways errors between the actual and demanded angles. In order toprevent this reducing the gain of the roll control law unnecessarily, asmall dead band can be introduced immediately after the absolute valueblock in the AGC unit. In order to provide rapid adaptation of thecontrol system to a rapid increase in ship motions, for example, if theship turns from being head-on to the prevailing wave direction to beingbeam-on, the AGC overload filter time constant K_(f) may be madenon-symmetrical. In other words, the value of K_(f) may be larger forpositive input to the K_(f) block, therefore, negative inputs. In thisway, an increase in the overload value (positive input to the K_(f)block) produces a rapid increase in the filtered overload value, while adecrease in the overload value (negative input to the K_(f) block)produces a slower reduction in the filtered overload value. In this way,the AGC section responds rapidly to the move excessive overloads, whilstpreventing rapid alteration of the roll control law gain.

It will also be understood that the AGC controller may be used forcontrolling a plurality of motions, i.e. more than 3 or 4 degress offreedom. In addition, it will be appreciated that other sensors may beused instead of accelerometers, for example, velocity sensors and themotions sensed may be linear or angular. In addition, although thesystem is described as being implemented in software, this is apreferred arrangement and it will be understood that the unitshereinbefore described can also be implemented in hardware by a personof ordinary skill in the art.

It will also be understood that where multiple loops are beingcontrolled simultaneously, for example, roll and pitch, the fact thatthe AGO unit makes the operation of the system more linear reduces theinteractions between the loops. In addition, the motions of the ship indifferent controlled axis can be measured and the available fin powerallocated according to the relative measurements in each axis. Thisallows the use of fin power to be maximised according to the seaconditions and the loading condition of the ship.

The system and apparatus hereinbefore described may be incorporated in avessel during construction or it may be retrofitted to existing vessels.In particular, the AGO controller may be interfaced with existing vesselcontrol system if the vessel control law used is capable of accepting avariable gain.

An advantage of the present invention is that the AGC controller allowsa stabilisation system designer to trade off performance against powerconsumption. It can provide better performance in rough weather than aconventional controller using the same drive system or it can providethe same performance with a smaller drive system. The AGC controller caneasily be extended to allow multiple control loops to use the same setof stabilisers with much reduced inter-loop interference compared withfixed gain controllers. This increases the performance of the controlloop. Furthermore, the AGC controller allows available stabilisationcapacity to be allocated between multiple control loops to try tomaximise overall stabilisation according to some pro-set criterion.

The AGC controller has applicability to stabilisation systems in avariety of ship and floating vessel designs, and although the AGCsub-system has the effect of reducing the control loop gain it actuallyreduces the shipper vessel motions below those achieved with theconventional control law because the phase lag between the demanded andthe actual fin angles is reduced and because the generation of theharmonics of the demanded fin angle is also much reduced.

We claim:
 1. A vessel motion stablisation system for stabilizing atleast one motion of a vessel having a plurality of actuatable vesselstabilising elements, said stabilising system comprising:motion sensingelements for sensing a parameter representative of the vessel motion tobe stabilised and generating a motion signal refecting said sensedparameter, control law means for receiving said motion signal and forprocessing said motion signal to provide an unlimited stabilisingelement demand signal, automatic gain control element for receiving asignal representative of said unlimited stabilising element demandsignal and for receiving a signal representative of an actualstabilising element condition, said automatic gain control elementincluding means for comparing the unlimited control law demand signalwith the signal representative of the stabilising element condition andproviding an error value, and signal processing means for processingsaid error value to provide automatic gain control value, means forcombining the gain control value with the unlimited demand signal toprovide a limited or actual demand signal for controlling the motion ofsaid motion stabilising element.
 2. A system as claimed in claim 1wherein said automatic gain control element includes absolute errordetecting means for determining the absolute error between thestabiliser demand signal and the actual stabiliser signal, filter meanscoupled to the output of the absolute error detecting means, said filtermeans having a filter input and a filter output and having a feedbackloop from the filter output to the filter input, filter gain controlmeans coupled to the filter output to provide a filter loop gain value,and gain limiting means coupled to output of said filter gain controlmeans for limiting the loop gain value of the automatic gain controlelement to a value between a maximum and a minimum.
 3. A system asclaimed in claim 2 wherein said gain limiting means limits a maximumdemand angle to a pre-determined value, for example, plus or minus 25°.4. A system as claimed in claim 1 wherein the vessel is a ship and thevessel stabilising elements are fins which are hydraulically orelectrically actuatable.
 5. A system as claimed in claim 4 wherein themotion detecting element are selected from the group consisting of:accelerometers, rate sensors and angle sensors, and the motioncontrolled is the roll motion of the ship.
 6. A vessel stabilisingsystem for stabilising the motion of a vessel, the vessel having aplurality of vessel motions to be controlled simultaneously, and thevessel having a plurality of actuatable vessel stabilising elements,each vessel stabilising element having one of a position sensing meansand a motion sensing means for providing an output signal representativeof the respective position or motion of the stabilising element, saidvessel stabilising system comprising:signal weighting means coupled tosaid sensing means for receiving said output signals representative ofthe position or motion of said stabilising elements, said weightingmeans including means for weighting the proportion of available power tobe set for a particular vessel motion control law according topre-determined criteria, and for generating weighted vessel motioncomponents, error detecting means coupled to each stabilising elementfor receiving and coupling a demand signal and an actual signal fromeach vessel stabilising element and for providing an error signal, errorsignal comparison means coupled to each of said error detecting meansfor receiving said error signals and for coupling said error signals andselecting the largest error signal as an output signal, output signalsprocessing means for filtering and amplifying said output signal toprovide a gain value between a maximum and a minimum, power allocationmeans coupled to the output of said weighting means and to said signalprocessing means to receive said weighted vessel motion components andsaid gain value, said power allocation means including summing means forsumming said weighted vessel motion components to provide a total value,and a plurality of vessel motion gain calculating means for determiningrespectively the gain of one of said respective vessel motions, saidgain calculating means receiving said total value of the weightedcomponents, the weighted component for a particular motion and the gainvalue between a maximum and a minimum, each gain calculating meansproviding a motion gain value which is fed to means to drive said motionstabilising element.
 7. A system as claimed in claim 6 wherein thevessel is a ship and the vessel stabilising elements are fins.
 8. Asystem as claimed in claim 7 wherein there are four fins which are usedby three control laws.
 9. A system as claimed in claim 6 wherein amotion sensor provides a motion signal from each controlled motion. 10.A system as claimed in claim 9 wherein the motion signals are weightedin accordance with control laws for each motion which depend on thevessel and installation-specific criteria.
 11. A system as claimed inclaim 10 wherein weighting is achieved by constants which are set at thedesign stage said constants being alterable in-situ to meet changingperformance requirements.
 12. A system as claimed in claim 6 wherein themaximum is 1.0 and the minimum is zero.
 13. A method of controlling atleast one motion of a vessel using a vessel stabilising system, thevessel having a plurality of vessel motions and vessel stabilisingelements, said method comprising the steps of,providing an automaticgain control unit to monitor a signal representative of the actualmotion of each stabilising element to be controlled, comparing theactual motion signal with a desired motion signal, and generating anautomatic gain control value, and adjusting the gain for each motionsignal by reducing the automatic gain control value to a level which thestabilising element can achieve.